It is known that static charges accumulating on the surface of materials can be dissipated by providing discharge points or by grounding. It is also known that the accumulation of static charges on or in flowing fluids, even flowing hydrocarbon fractions such as gasoline, can be prevented by grounding of the transfer lines and/or by incorporating "anti-static" compounds or solutes in said fluids. However, such measures for dissipating accumulated or preventing the accumulation of static charges are not feasible for use with particulated solids which must be kept uncontaminated. Also such measures as providing point discharges or grounding are not mechanically feasible for particulate solids, especially crystalline materials which have thousands of particles per unit volume. Such particulate solids can in flow transfer pass over grounding ribbons, wires, etc. with only a minor amount of the particles in contact with such means for grounding dissipation of the static charge. Then there is the problem of providing in some manner the particles with point discharges from particle to particle to some means for charge draw-off so that the point discharges are not destroyed by abrasion as the particles are transferred by fluidized flow.
We have discovered that particulate solids having smooth discrete particles, for example spherical or cylindrical particles, rather than sharp cornered and/or sharp ended particles when transferred as dry product from a drying step to storage, or from storage to shipment or from shipping container to reaction, formulation, packaging, and the like by pneumatic fluidization can accumulate static charges. Such accumulated static charges can cause agglomeration or surface build-up of the particles by physical attraction of dissimilar charges to the extent that the transfer line conveying the particulate solid product becomes restricted to flow and even plugged.
Such flow restriction of transfer line was first noticed when a particulate solid product having smooth discrete particles was being transferred by pneumatic fluidization from a closed hopper car through a metal tube or pipe transfer line having an internal diameter of thousands of times larger than the diameter of the particles. Said product during its manufacture to storage prior to shipment had previously been conveyed from drying to storage and storage to bagging or tote bins and bags for shipment had been transferred as a pneumatically fluidized mass without evidence of static charge build-up or plugging caused thereby.
It was only after use of hopper car shipment that the flow restriction problem occurred during the pneumatic transfer of the product through a rather small, about 10 cm internal diameter, metal transfer line. Such flow restriction caused hopper car unloading time to increase by a factor of 2 to 3 over normal unloading time. Whether the flow restriction was caused by static charge accumulation during manufacture or loading for shipment or only during transfer through the metal line is not known. There were ample contacts of the particles with materials of dissimilar dielectric properties to cause generation of the static charge during manufacture (drying to storage) or shipment (storage to hopper car). But such pneumatic fluidized transfers were through larger diameter metal lines. However, a competitor's product of similar particle size distribution did not suffer the same flow restriction when being unloaded from the closed hopper car by pneumatic fluidization therein and transfer through the metal line of smaller, about 10 cm internal diameter.
Our observation was not of a new phenomenon. Rather, transfer line plugging during pneumatic transport caused by high static charging of the particles was reported in Powder Technology, 6 (1972) pages 283-294 by S. L. Soo and S. K. Tong.